This invention was conceived in connection with apparatus for the manufacture of uncreped stretchable paper as disclosed in Cluett U.S. Pat. No. 2,624,245; Welsh et al., U.S. Pat. No. 3,011,545; and Schaefer, U.S. Pat. No. 3,329,562, all owned by a common assignee of those patents and the present invention. Although utility of this invention is not limited to manufacture of uncreped stretchable paper, the invention is shown illustratively and is described herein as applied to that use.
Making an uncreped longitudinally stretchable paper web according to the teachings of these patents is accomplished by shrinking the paper web longitudinally by means of forces parallel to the surfaces of the web while the web is in a suitably moist condition and while the web is maintained under sufficient pressure normal to its surface to prevent creping or crinkling.
An accepted method of providing parallel forces to a web incapable of self support for the purpose of shrinking the web is to feed the web to a segment of travel between a rotating drum and a traveling endless resilient belt. The belt's outside surface is accelerated prior to the point of web feed, and then the belt's outside surface is caused to decelerate after the point of web feed. The deceleration of the belt's outside surface while it is in contact with the web causes the parallel forces on the web which compact or shrink the web.
More specifically, as described in U.S. Pat. Nos. 2,624,245 and 3,011,545, the acceleration and subsequent deceleration of the outside surface of the resilient belt may be effected by feeding the belt along its travel to a restricted nip space formed by and between a loaded compactor bar and a rotating smooth faced drum. As the resilient belt is substantially noncompressible, it accelerates prior to the nip space in order to pass through the restriction of the nip space and it decelerates after it passes through the nip space. The web, which is fed into the nip space and into contact with the outside surface of the traveling belt after the belt has been accelerated, is shrunk by the parallel forces transmitted to it as the outside surface of the belt decelerates and while the web is pressed against the drum. The shrinkage of the web is controlled by varying the restriction of the nip space which causes change in surface velocity of the outside surface of the traveling belt.
In commercial practice, even though the compactor may be a bar rotating type, there tends to be a scuffing effect on the inside surface of the belt if the belt runs dry. The use of a lubricant of water or other liquid eliminates this scuffing and it has been observed that the compactor bar even stops rotating when lubricant is present. With a non-rotating bar, which is more commonly used, the need for a lubricant is essential but presents the following situation. If the amount of lubricating liquid supplied, such as by spraying on the belt, is carefully adjusted so that it equals the loss consumed during lubrication and by evaporation, etc., then the inner belt surface is adequately lubricated in the nip area and there is no surface liquid available to leak out of the nip at the ends of the belt to cause damage to the paper web at those points. It can be readily seen that such a nice adjustment is difficult in practice, however and usually to avoid the nip running dry, an excess of water at the belt approach side of the nip is provided. This further requires that excess water in the puddle be removed, and such removal has been achieved in the past by the fitting of edge pans to the edge of a belt such that the excess water can flow into the pan and be allowed to run away. Because these edge pans have from the very beginning proven to be difficult to design with an effective seal, they have never been very popular. In practice, they leak badly, causing trouble to the edges of the paper web and, from time to time, they damage the rubber belt. To obviate this, many users of equipment have removed these edge pans and have fitted compressed air pipes so that jets of air can be directed at the edges of the belt where the water would normally leak away and this air stream creates an air dam which restricts the leakage of water at this point. However, this is only effective if the pool formed by the excess water at the belt approach side of the compactor bar remains at a constant height. For the reasons given beforehand, this state of affairs cannot usually be kept and, the pool frequently rises to a level whereby it overcomes the air resistance of the air dam and leaks around the belt edges as before. Thus, the main contribution of the new compactor bar of this invention is to maintain a constant but carefully controlled pool height so that, once an air dam or edge pans are adjusted to contain the water at the edges of the belt, the state of equilibrium will not change.